This invention relates to a method and system for producing poly(lactic acid) while recovering lactide discharged as vapor from a polymerization reaction vessel in which the lactide is subjected to ring-opening polymerization.
Poly(lactic acid) is a polymer which is able to decompose in a living body, and has been used in the medical field owing to its excellent mechanical properties. Also, poly(lactic acid) is able to decompose in a natural environment. Accordingly, it has been hoped to provide a way of using poly(lactic acid) in applications for environmental protection.
Poly(lactic acid) can be synthesized by polycondensation of lactic acid monomers, or by ring-opening polymerization of lactide dimers. Low molecular weight polymers can be easily produced by the former method. However, high molecular weight polymers is difficult to be synthesized other than the latter method.
In the case of ring-opening polymerization, there is no need to reduce the pressure inside the reaction vessel as a condition for advancing the polymerization reaction. The polymerization temperature of the polymerization reaction vessel can be regulated by vaporizing lactide and discharging it out of the reaction vessel as vapor during the course of the polymerization reaction. Furthermore, it is desirable to remove any unreacted lactide remaining around the poly(lactic acid) at the end of the polymerization reaction. A vacuum apparatus is used to reduce the pressure inside the reaction vessel and remove the lactide vapor. However, the boiling point of lactide is about 250.degree. C. In order to remove the lactide from the reaction vessel as vapor, it is necessary to maintain the discharge line at a temperature equal to or greater than the boiling point. Moreover, the melting point of lactide is about 97.degree. C. Any cooling of lactide to a temperature lower than the melting point results in lactide becoming solidified and causing blockages in the vacuum apparatus or other parts.
Furthermore, recovering of raw material lactide will increase the yield of poly(lactic acid). For this reason, it is desirable to positively recover lactide rather than discharging it out of the reaction vessel. However, there has been no established technique for recovering lactide vapor discharged in the process of producing poly(lactic acid).
Japanese Unexamined Patent Publication No. Hei 7-309863 discloses a method in which lactide is recovered by using a vacuum pump to suck up lactide produced by decomposition of poly(lactic acid) and introduce it into a trap for trapping lactide. Although no specific trapping method or temperature conditions are disclosed, the publication suggests that lactide could be trapped by filling the trap with an alcohol. However, when lactide is recovered using an alcohol, it becomes necessary to include a refining process for separating the lactide from the alcohol before reuse of the recovered lactide. This requires complicated equipment.
There has therefore been a demand for novel poly(lactic acid) production method and system which make it possible to trap and recover lactide vapor and reuse recovered lactide vapor as raw material without using a solvent such as alcohol.